By Athena Hessong • Updated Mar 24, 2022

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Relative humidity indicates the proportion of moisture present in the air compared to the maximum amount it could hold at a given temperature. Because warmer air can retain more moisture than cooler air, relative humidity varies with temperature. Accurately measuring relative humidity with a simple wet‑dry bulb setup allows you to determine whether your environment is too dry or too humid. Excess moisture can promote mold growth, while low humidity can dry skin and exacerbate respiratory irritation. By managing indoor humidity with dehumidifiers or humidifiers, you can mitigate these issues.

Step 1 – Position the Thermometers

Attach two mercury‑filled bulb thermometers side by side to a piece of cardboard using tape, ensuring the bulb tips extend beyond the edge of the cardboard.

Step 2 – Prepare the Damp Cloth

Submerge a facial tissue or a 3‑inch square of cotton cloth in water. Squeeze out the excess liquid so the cloth remains moist but not dripping.

Step 3 – Apply the Damp Cloth

Wrap the damp cloth around the bulb of one thermometer. Keep the other thermometer dry.

Step 4 – Record Temperatures

After 10 minutes, read both thermometers. The dry thermometer shows the ambient air temperature, while the wet‑bulb thermometer reflects the evaporation temperature.

Step 5 – Convert to Celsius

For each Fahrenheit reading, subtract 32 and multiply by 5⁄9. For example, 50 °F → (50 – 32) × 5⁄9 ≈ 10 °C.

Step 6 – Calculate the Temperature Difference

Subtract the wet‑bulb temperature (°C) from the dry‑bulb temperature (°C).

Step 7 – Consult the Humidity Chart

Locate the row corresponding to the dry‑bulb temperature on the left side of the relative humidity chart.

Step 8 – Find the Temperature Difference Column

Scan the top row of the chart to identify the column that matches the temperature difference calculated in Step 6.

Step 9 – Read the Relative Humidity

The intersection of the chosen row and column gives the relative humidity percentage.

Materials Required

• Two mercury‑filled bulb thermometers
• Cardboard
• Tape
• Facial tissue or a 3‑inch square of cotton cloth
• Wet‑dry bulb relative humidity chart (see Resources)